Sn=1/1·4 + 1/4·7 +...+1/(3n-2)(3n+1)

1+2+3+4+.+n,求Sn 已知数列{an}的前n项和为Sn=1+2+3+4+…+n,求f(n)= Sn /(n+32)Sn+1的最大值 数列求和：Sn=1/1*2*3+1/2*3*4+.+1/n*(n+1)*(n+2) 求Sn 求和：Sn=1*2*3+2*3*4+……+n(n+1)(n+2) Sn=n(n+2)(n+4)的分项等于1/6[n(n+2)(n+4)(n+5)-(n-1)n(n+2)(n+4)]吗? 已知等差数列{an}的前n项和为Sn,且(2n-1)Sn+1 -(2n+1)Sn=4n²-1(n∈N*) 求和Sn=1/1*4+1/4*7+.1/(3n-2)(3n+1) 设Sn为数列{an}的前n项和,且有S1=a,Sn+Sn-1=3n²,n=2,3,4,. Sn=3+2^n Sn-1=3+2^(n-1).则Sn-Sn-1=? 求和sn=1*2+4*2^2+7*2^3+...+(3n-2)*2^n Sn=1+ 4/5 + 7/5² +……+ (3n-2)/5^(n-1) 数列Sn=(3n+1)/2-(n/2)an